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#### David Griffith

- Jan 1, 1970

- 0

I am trying to construct a variable power supply from the schematic

listed here.

http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#317.gif

Basically I wanted to know if the author has got the value for the

filter capacitor correct or not, as it seems very high (36 millifarads)

I have tried plugging in the maths and it gives me a more realistic

value, but I would appreciate any insight into which of us has got it

wrong...

"Electronic Devices (2nd Ed)" by Thomas L Floyd gives the equation for a

filter capacitors size to be roughly as follows

C = 0.0024 / (R * r)

where R is the load resistance and r is the 'ripple factor' or ripple

voltage over DC voltage.

The transformer I am using is not marked, when connected to a supply

measuring 240V rms exactly on my DVM measures 13.0V rms output on the

secondary.

I therefore take the peak voltage to be 13 * sqrt(2) = 18.4V peak

The LM317T regulator seems to require r to be 1/18.4 or less giving a

maximum actual ripple of 1V , and can output up to 1.5A setting R to be

V/I = 18.4/1.5 = 12.3 ohms

C = (0.0024 * 1.5) after cancelling voltage from both sides of the

equation = 3600uF not 36,000uF as stated in the schematics.

Is my maths sound?

Thanks in advance

David Griffith